EP 0908278 B1 discloses a method for determining the quantity of clippings produced by an electrically driven cutting tool, wherein this determination is based on the notion that the current consumption of a d.c. motor driving the cutting tool is subjected to fluctuations that are caused by the individual cutting processes. These current fluctuations are measured, evaluated and used as a measure for the quantity of the produced clippings. If this method is realized in a hair cutting device, an indicating device is also activated after a certain reference value for the quantity of the produced clippings is reached in order to inform the user of the necessity to clean the hair cutting device.
DE 10229319 A1 discloses a method for controlling an oscillating electric motor that serves, for example, for driving an electric razor or an electric toothbrush. In this method, a coil of the electric motor is supplied with an electric current at a certain point in time in order to realize a magnetic field that originates from a first motor component and acts upon a second motor component in such a way that the second motor component carries out an oscillating movement relative to the first motor component. At the time, at which the coil is supplied with a current, at least one electric parameter of the coil is determined that is correlated with the state of motion of the first or the second motor component, for example, the voltage applied to the coil and/or the current flowing through the coil and/or the change of the current flowing through the coil as a function of the time. The future current supply to the coil, i.e., the control of the oscillating electric motor, takes place in dependence on the determined parameter(s). In addition, DE 10229319 A1 describes how the oscillation frequency and the oscillation amplitude of the motor components can be determined.